Study to Help Improve Rainfall Forecasts for West Coast

Mar 01, 2007

Researchers from the National Oceanic & Atmospheric Administration (NOAA) Earth System Research Lab are monitoring air, water and soil in the American River basin between Reno, Nev., and Sacramento, Calif., through the end of March.

Working closely with NOAA National Weather Service forecasters and hydrologists, scientists are improving predictions of California's heavy winter rains to help water resource managers prevent catastrophic flooding in the Sacramento region. New sensors, computer models and other tools tested during the study, called the Hydrometeorology Testbed (HMT), eventually will be used to improve NOAA National Weather Service rainfall forecasts up and down the West Coast.

Prolonged winter rains along the American River basin help make Sacramento one of the nation's most vulnerable urban areas. Damaging floods during the winter of 1986 and again in 1996 to 1997 drew attention to the threat of catastrophe. County flood maps show parts of Sacramento submerged under 20-plus feet of water in scenarios where multiple levees break. To prevent such disasters, NOAA wants to provide water managers with highly credible warnings and enough time to prepare.

"We want a reliable prediction so we can say with confidence, 'A week from now we're expecting a flood that will significantly threaten the levees,'" HMT project manager Tim Schneider said.

HMT has deployed an assortment of radars, wind profilers, rain and snow gauges, automated weather stations using Global Positioning System technology and other sensors along the North Fork of the American River, turning the river basin into one of the most monitored areas in the country. During intensive observing periods conducted through March, daily weather balloon launches will increase to every three hours instead of the usual 12.

The American River flows through the Folsom Dam, which holds up to a million acre-feet of water upstream from Sacramento. An acre-foot is the amount of water required to cover an acre one foot deep. The dam's reservoirs provide the state with water for irrigation, hydropower, wildlife habitat, recreation and other uses. Resource managers balancing these conflicting demands with public safety are among those most in need of better rainfall forecasts.

HMT studies of West Coast climate patterns recently revealed a phenomenon now called "atmospheric rivers" (ARs). These long plumes of moisture streaming over the Pacific Ocean bear 95 percent of the water vapor transported each year from the tropics to the mid-latitudes. ARs are responsible for heavy winter rains and major floods in California, Oregon and Washington. NOAA Earth System Research Lab scientist Marty Ralph is devising an intensity scale similar to those used for hurricanes and tornadoes to more easily identify the strongest ARs and warn of possible floods.

As Earth's climate changes, ARs are expected to become even more powerful, said Ralph. A warmer atmosphere will hold more water vapor, much of it concentrated in these plumes, which are likely to travel farther north as the Arctic warms.

"With climate change, the West Coast is expected to experience even more extreme winter rainfall than we've seen so far, along with extreme episodes of dry, hot weather in summer. Highly accurate rainfall forecasts with longer lead times will be essential," he said.

In an earlier experiment along the Russian River in California, NOAA found that every major flood during the study period was associated with an atmospheric river. Storms fueled by ARs dumped twice as much snow on coastal mountain ranges as all other storms combined. Between 20 and 35 ARs hit the West Coast each winter. More than 100 reach the coast in summer but because the relative humidity is lower and they are more diffuse, summertime ARs bring little rainfall.